Synchronism control system



NOV. 28, s CRARY 2,363,833

SYNCHRONISM CONTROL SYSTEM Filed June '11, 1943 u mm nu incur 0FInventor: Selden B. Gravy,

/ B H is Attorney.

Patented Nov. 28,1944

SYNCHRONISM ooN'rRor. SYSTEM Selden B. Crary, Schenectady, N. Y.,assignor to General Electric Company, a corporation of New YorkApplication June 11, 1943, Serial No. 490,461

19 Claims.

This invention relates to control systems and more particularly toimprovements in automatic systems for causing synchronousdynamo-electrlc machines to regain synchronism.

It is well known that when two or more synchronous dynamo-electricmachines are connected tothe same circuit there is a definitesynchronizing torque acting to hold the machines in synchron-ism. Ingeneral this torque .isproportional to the excitation of the machines.

However, occasionally the mechanical or shaft torque of amachine exceedsthe necessary restoring or synchronizing torque following a disturbance,in which case the machine will pull out of step with the system orcircuit to which itis connected. For example, the machine may in certainoperating conditions or following a disturbance pick up so muchelectrical load that its prime mover cannot supply it or keep up with.it, in which case the machine falls out of step below synchronous speedor below the speed of the other machines of the system.

I have discovered, however, that when the machine 'is operatingsubsynchronously in the above-described manner increasing the excitationto high values does not promote the regaining of synchronisin and, infact, I have found that it is exactly the wrong thing to do. This isbecause the increase in excitation increases the armature current andthis increases the load supplied at the frequency of the rotating fieldand produces a braking torque, which braking torque is proportional tothe square of the excitation. I

In accordance with this invention, I provide an automatic system inwhich, when the machine operates below synchronism, the excitation isautomatically reduced and the mechanical shaft torque is automaticallyincreased in the speed increasing direction. v

In accordance with the preferred embodiment of, the invention there isalso provided automatic means responsive to overspeed or supersynchronous operation for increasing the excitation and decreasing theshaft torque.

An object of the invention is to provide a new 3 and improved automaticcontrol system.

companying drawing, wherein there is illustrated diagrammatically anembodiment of. the invention, there is shown asynchronous'dynamoelectric generator I comprising a three-phase armatureor stator winding 2 and a direct-current field orv exciting winding 3mounted on a rotor which includes an amortisseur winding 4. The rotor isdriven by a prime mover 5 shown, by way of example, asfa steam turbine.The armature winding I2 is connected to'a three phasealternating-current power system 6 to which one or more othersynchronous machines (not shown) are connected.

The speed torque characteristic .of' the turbine 5 is controlled by anysuitable means, such as a centrifugal governor 1 driven by bevel gears-8 fromthe turbine shaft. This governor is connected by a linkage 9 toa'steam inlet valve Ill. The normal fulcrum. for the linkage 9 is at I land the action of the governor. is such that when the turbinespeedincreases the left-hand end of the link 8 moves downwardly (in thedrawing), thereby to close the valve Ill and when the turbine speeddecreases the link moves upwardly thereby to open'the steam valve.

In order to adjust the power output of the turbo-generator the positionof the fulcrum II is adjusted up and down by means of a so-calledsynchronizing motor I2. This is shown by way of exampleas a simple splitfield motor and it is connected to move the point It up or down by meansof a gear l3 on its shaft which meshes with .a gear H which rotates asupport ii for the fulcrum It. This" support is internally threaded andis engaged by the threads of a stud l6 whose lower end is fixed at IT.The result is that rotation of the m'emberlS causes it to move up anddown on the-threads of f6 thereby to raise and lower the fulcrum H; Thisobviously changes the position of the steam valve and therefore changesthe power output of the generator I which so long as'it'remainsinsynchronism will'not change its speed'and consequently the position ofthe centrifugal governor I will not be affected. r a 1 In order that theloading of the turbine generator may be adjusted to any particular valuethe free terminals of its split field are connected to rheostats Hand [9respectively. The rheoand the free terminals of the rheostats areconnected to the negative side of this source. In this manner bothhalves of the split field are constantly energized and for any settingof the manually operated rheostat IS the motor will run until it adjuststhe rheostat I8 to a resistance value equal to that of the rheostat I9at which point the current in the two halves of the field will be equaland the motor will come to rest. Thus, the load may be increased ordecreased by adjusting the rheostat I9 in one direction or the other.The opposite ends of the split field are labeled and C indicatingopening and closing directions respectively; that is to say, if the halfof the split field labeled 0 carries more current than the half labeledC the turbine valve will tend to open and, conversely, if the half ofthe split field labeled C carries more current than the other half thevalve will tend to close.

The direct current for energizing the field winding'3 may be obtained inany suitable manner and, as shown by way of example it is obtained froma self-excited shunt-connected exciter 2| whose terminals are connectedto the field winding 3 through slip rings 22.

For detecting asynchronous operation of the generator I there isprovided an out-of-step relay 23 of any suitable type. This relay isprovided with a set of normally open contacts 24 which close when themachine has definitely lost synchronism and is definitely operatingasynchronously. The relay 23 responds to the three phase currents in thesystem 6 by means of three current transformers 25 and it responds tothe phase voltages of the system by means of suitably connectedpotential transformers 26. A suitable form of out-of-step relay fordiscriminating be tween asynchronous operation on the one hand and loadswings or oscillations or fault conditions on the other hand isdescribed and claimed in Patent 2,095,117 granted October 5, 1937, on anapplication of E. H. Bancker et al. and assigned to the presentassisnee.

For discriminating between overspeed or supersynchronous operation andunderspeed or subsynchronous operation there is provided adifferentialspeed responsive device 21. This device is similar to a three-phasewound rotor induction motor or a differential selsyn device in that ithas a polyphase stator winding 28 and a polyphase rotor winding 29. Therotor is shown excited by the voltage of the circuit 6 through thepotential transformer 26 and the stator 28 is excited from a tachometergenerator 30 driven by the turbine 5. This tachometer generator is shown'by way of example as a permanent magnet excited machine. The rotor ofthe differential speed device 21 carries a movable contact 3| forcooperation with a pair of fixed contacts 33 and 34. When the machine Iis in synchronism with the system the rotating magnetic fields of thewindings 28 and 29 are in synchronism so that there is no torque tendingto turn the rotor of the device 21 beyond the normal range ofsynchronousangular displacement and consequently the contact 3I stays within itsnormal operating range. However, if the machine I loses synchronism thefrequency of the output of the generator 30 either increases ordecreases with respect to the frequency of the system so that there willbe a slip frequency or differential frequency between the fields of therotor and stator of the device 21 and this will develop a torque causingthe contact ill to move toward either the contact 33 or the contact 34and if this differential speed continues contact will be made.

Such contact, if sustained, is a reliable indication of whether thedeviation in speed is an overspeed or an underspeed condition but it isnot as reliable an out-of-step indication as the contacts 24 of theout-of-step relay 23 because the machine I can oscillate through a;relatively wide angle with respect to the system 6 without actuallylosing synchronism although such oscillation might cause closing of thecontacts of the differential device 21.

In order to restore machine I to synchronism in case it losessynchronism a pair of control circuits are completed selectively throughthe contact of the out-of-step relay 23 and the differential speed relay21 in series and these circuits serve to actuate auxiliary relays 35 and36. Thus, the circuit for the relay 35 is completed from the positiveside of the control source through the contacts 24 and the contacts3I--33 in series and through the operating winding of the relay 35 tothe negative side of the control source. The other control circuit iscompleted through the contacts 24 and the contacts 3I-34 in series.

The relay 35 which picks up as a result of subsynchronous operation ofthe machine I controls circuits which cause the throttle of the turbine5 to open and cause the excitation of the machine I to decrease. Thus,it is provided with a set of normally open contacts 31 which serve toconnect the negative side of the control source directly to the O oropening terminal of the split field of the synchronizing motor and it isprovided with a set of normally closedcontacts 38 which short circuit aresistor 39 in the field circuit of the exciter 2|. Likewise, the relay36 which picks up on overspeed or supersynchronous operation of themachine I controls circuits which reduce the shaft torque of the turbine5 and which increase the excitation of the machine I. Thus, it isprovided with a set of normally open'contacts 40 which serve to connectthe negative side of the control source directly to the C or throttleclosing terminal of the split field of the synchronizing motor and it isprovided with another set of normally open contacts H which serve whenclosed to short circuit a resistor 42 in the field circuit of theexciter 2|. I

In many cases it will be desirable to provide the machine I with anautomatic voltage regulator and a suitable form of regulator isillustrated.

in the drawing as comprising a torque motor 43 responsive to the circuitvoltage. The rotor of this torque motor is connected to a contactcarrying yoke 44 on which are mounted raise and lower contacts whichcooperate respectively with a center contact 45. The torque of the motoris balanced by a spring 46. The raise and lower contacts are connectedrespectively to the raise and lower terminals of the split field of areversible motor 41 for operating a rheostat 43 in the field circuit ofthe exciter 2|. In order to prevent a conflict between the operation ofthe voltage regulator and the operation of the synchronism restoringmeans the relays 35 and 35 are provided with additional normally closedcontacts 48 and 50 which are respectively in the raise and lowercircuits of the voltage regulator. Consequently, when the relay 35 ispicked up so as to cause a decrease in excitation the raising circuit ofthe regulator is opened and similarly an increase in excitation thelowering circuit of v the regulator is opened.

'I'he operation oi the illustrated embodiment of the invention, will beobvious from the pre- -ceding description. It is to be noted, however,

that as soon as synchronism is restored and the relays 35 and 36 dropout the synchronizing motor l2 will'again be under the control of therheostats l8 and [50 that the motor will return the throttle andconsequently will return the. loading of the machine I to the conditionparticular embodiment of this invention, it will be obvious to thoseskill'edin the art that various changes and modifications can be madetherein without departing from the invention and, therefore, it is aimedinthe appended claims to cover all such, changes and modifications asfall within the true spirit and scope of the invention. a

What I claim as new and desire to secure'by Letters Patent of the UnitedStatesis: V

1. In a synchronous to-synchronous power system, a synchronousdynamo-electric machine, means for controlling the shaft torque of saidmachine, means for controlling the excitation of said machine, and meansresponsive to supersynchronous operation of said machine for causingsaid torque controlling means to provide a deceleration producing changein torque and for causing said excitation controlling means to increasethe excitation.

2. In a synchronous-to-synchronous power system, a synchoronousdynamo-electric machine, means for controlling the shaft torque of saidmachine, means for controlling the ex citation of said machine, andmeans responsive to subsynchronous operation of said machine for causingsaid torque controlling means to provide an acceleration producingchange in torque and I 9,868,838 when the relay 88 is picked up so as tocause machine for causing said torque controlling means to provide adeceleration producing change in torque and for causing said-excitationcontrolling means to increasethe excitation regardless of the action ofsaid regulator.

5. In 'a synchronous-to-synchronous power system, asynchronousdynamo-electric machine,

means forcontrolling the shaft torque..of said machine, means forcontrolling the excitation of said machine, an automatic regulatorof anelectrical condition of said machine for operating said excitationcontrolling means. and means responsive to subsynchronous operation ofsaid machinefor causing said-torque controlling means to provide .anaccelerationproducing change-in torque and forwcausing saidexcita-' tioncontrolling means-to decrease the excitation regardless of the action ofsaid regulator.

6. In a synchronous to synchronous,.,-,power system, a synchronousdynamo-electric.machine, means for controlling the shaft torque ofdsaidmachine, means for controlling the excitation of said machine, andautomaticvoltage regulator for operating said excitation, controllingmeans, means responsive to supersynchronous. operation of said machinefor causing said torquecontrolling means to provide. a decelerationproducing change in torque and-forcausiifi said: excita- .tioncontrollingmeans to increase the excitation,

and means responsive to subsynchronousoperation of, said machine ,forcausing said.' torque controlling means to, provide anaccelerationproducing change intorque and for cau'sirig said excitationcontrolling means tode'creasethe exfor causing said excitationcontrolling means to decrease the excitation 3. In asynchronous-.to-synchronous power system, a synchronous dynamo-electricmachine,

means for controlling the shaft torque of said machine, means forcontrolling the excitation of said machine, means responsive tosupersyn-.

chronous operation of said machine for causing. said torque controllingmeans to provide a deceleration producing change in torque and forcausing said excitation-controlling means to increase the. excitation,and means responsive to subsynchronous operation of said machine forcausing said torquecontrolling means to pro-- vide an accelerationproducing change in torque and for causing said excitation controllingmeans to decrease the excitation.

-- said machine. and means responsivept-o, sub- 4. In asynchronous-to-synchronous power system, a synchronousdynamo-electricmachine,

means for controlling the shaft torque of said machine, means, forcontrolling the excitation of said machine, an automatic regulator of anelectrical condition of said machine for operating said excitationcontrolling means, and means responsive to supersynchronous operation ofsaid citation regardless of the action of "3a later. i: I

7. In a synchronous-to-synchronous power system, a synchronousdynamo-electric machine. means for controlling the shaftto'rque-"of saidmachine. means for controlling-the excitation of said machine, and meansresponsive'tdsupersynchronous operation of said machine for causing saidtorque controlling means to p rovide a deceleration producing changeintorque-and for causing'said excitation" controlling m'can-s'to increasethe excitatiohjsaid means responsive to supersynchronous operationincluding means responsive to out-of-step operation of saidmachine andmeans for discriminating "between overspeed and underspeed operation'pfsaid machine.

8. In a synchronous-to+synchronous power system, asynchronous.dynamo-electric machine, means for controlling the shafttorque of said machine. means for controllingthe'excitation ofsynchronous-operation of said machine for-causingsaid torquecontrollingmeans to-provide an acceleration producing change in torque and forcausing said excitation controlling means to decrease theexcitation-said means responsive to subsynchronous operation includingmeans responsive to out-of-step operation ofsaid machine and means fordiscriminating between overspeed and underspeed operation of saidmachinea 9. ,In a synchronous-to-synchronous power system, a synchronousdynamo-electric. machine.

means for controlling'theshaft torque of said machine, means forcontrolling ,the excitation of. said machine, means responsive tosupersynchronousioperation of said machine for causing said torquecontrolling means toprovide a deceleration producing change in torqueand for causing said'excitation controlling means to increasetheexcitation, and means responsive to' mbaynchronous operation of saidmachine for causing said torque controlling means to provide anacceleration producing change in torque and for causing said excitationcontrolling means to decrease the excitation,v said means responsive tosupersynchronous operation including means respons'ive toout-of-stepoperation of said machine and means for discriminatingbetween overspeed and underspeed operation of said machine.

10. In a synchronous alternating-current power system, a synchronousgenerator, a prime mover therefor, means for controlling the excitationof said generator, means for controlling the torque of said prime mover,and means responsive to supersynchronous operation of said generator forcausing said excitation controllingmeans to increase the excitation andfor causing said torque controlling means to decrease the torque while-maintaining said generator connected to the systom.

11. Ina synchronous alternating-current power system, a synchronousgenerator, a prime mover therefor, means for controlling the excitationof said generator, means for controlling the torque of said prime mover,and means responsive to subsynchronous operation of said generator forcausing said excitation controlling means to reduce the excitation andfor causing said torque controlling means to increase the torque whilemaintaining said generator connected to the system.

12. In a synchronous alternating-current power system, a synchronousgenerator, a prime mover therefor, means for controlling the excitationof said generator, means for controlling the torque of said prime mover,means responsive to supersyn'chronous operation of said generator forcausing said excitation controlling means to increase the excitation andfor causing said torque controlling means to decrease the torque while.maintaining said generator connected to the systern. and meansresponsive to subsynchronous operation of said generator for causingsaid excitation controlling means to reduce the excitation and forcausing said torque controlling means to increase the torque whilemaintaining said generator connected to the system.

13. In a synchronous alternating-current power system, a synchronousgenerator, is prime mover therefor, means for controlling the excitationof said generator, an automatic regulator of an electrical condition ofsaid machine for operating said excitation controlling means, means forcontrolling the torque of said prime mover, and means responsive tosupersynchronous operation said generator for causing said excitationcontrolling means to increase the excitation and for causing said torquecontrolling means to decrease the torque regardless of the action ofsaid regulator while maintaining said generator connected to the system.

14. In a synchronous alternating-current ower system, a synchronousgenerator, a prime mover therefor, means for controlling the excitationor said generator, an automatic regulator of an electrical condition ofsaid machine for operating said excitation controlling means, means forcontrolling the torque of said prime mover, and means responsive tosubsynchronous operation of said generator for causing said excitationcontrolling means to reduce the excitation and for causing said torquecontrolling means to increase the torque regardless of the action ofsaid reguaseassa later while maintaining said generator connected to thesystem.

15. In a synchronous alternating-current power system, a synchronousgenerator, a prime mover therefor, means for controlling the excitationof said generator, an automatic voltage regulator for operating saidexcitation controlling means, means for controlling the torque o! saidprime mover, means responsive to supersynchronous operation of saidgenerator for cauling said excitation controlling means to mcrello theexcitation and for causing said torque controlling means to decrease thetorque, and means responsiveto subsynchronous operation of saidgenerator for causing said excitation controlling means to reduce theexcitation and for causing said torque controlling means to increase thetorque regardless of the action of said regulator while maintaining saidgenerator connected to the system.

16. In a synchronous altemating-current power system, a synchronousgenerator, a prime mover therefor, means for controlling the excitationof said generator, means for controlling the torque of said prime mover,and means responsive to supersynchronous operation or said generator forcausing said excitation controlling means to increase the excitation andfor causing said torque controlling means to decrease the torque, saidmeans responsive to supersynchronous operation including meansresponsive to out-'of-step operation of said generator and means fordiscriminating between overspeed and underspeed operation of saidgenerator while maintaining said generator connected to the systerm.

17. In a synchronous alternating-current power system, a synchronousgenerator, a'prime mover therefor, means for controlling the excitationof said generator, means for controlling the torque of said prime mover,and means responsive to subsynchronous operation of said generator forcausing said excitation controlling means to reduce the excitation andfor causing said torque controlling means to increase the torque, saidmeans responsive to supersynchronous operation including meansresponsive to out-of-step operation of said generator and means fordiscriminating between overspeed and underspeed operation or saidgenerator while maintaining said generator connected to the system.

18. In a synchronous alternating-current power system, a synchronousgenerator, a prime mover therefor, means for controlling the excitationof said generator, means for controlling the torque of said prime mover,means responsive to supersynchronous operation of said generator forcausing said excitation controlling means to increase the excitation andfor causing said torque controlling means to decrease the torque, andmeans responsive to subsynchronous operation of said generator forcausing said excitation controlling means to reduce the excitation andfor causing said torque controlling means to increase the torque, saidmeans responsive to supersynchronous operation including meansresponsive to out-of-step operation of said generator and means fordiscriminating between overspeed and underspeed operation of saidgenerator while maintaining said generator connected to the system. I

19. In combination, a synchronous alternatingcurrent power system, asynchronous generator connected thereto, a prime mover for saidgenerator', a rlicostat for controlling the excitation of saidgenerator, a governor synchronizing motor for controlling the torque ofsaid prime mover, an automatic voltage regulator for said generator forcontrolling said rheostat, an out-0fstep relay for responding toasynchronous operation of said generator, diiferential speed responsivemeans for discriminating between over and underspeed operation of saidmachine with respect to the rest of the system, means jointly responsiveto said relay and differential speed means when said machine isoperating supersynchronously for operating said rheostat to increase theexcitation regardless of the action of said regulator and for operatingsaid motor to reduce the torque of said prime mover, and means jointlyresponsive to said relay and differential speed means when said machineis operating subsynchronously for operating said rheostat to decreasethe excitation regardless of the actio of said regulator and foroperating said 10 motor to increase the torque of said prime mover.

SELDEN B. cRhRY.

